1. Field of the Invention
This invention relates to gauge calibration. In particular, but not exclusively, it relates to the calibration of gauges for measuring the weight/thickness of coatings such as lacquer applied to metals such as tinplate used in the can making trade.
2. Description of Prior Art
Capacitance based film weight gauges are known which are used for the measurement of dry film “weight”. Whilst measurements taken by such known gauges are accurate, their calibration and stability over time is poor. In these gauges, a panel of known film weight is used to programme a selected channel to give the correct film weight when a measurement is taken. The panel allows the meter to be programmed with a factor K which relates the capacitance measured by the gauge to the film weight. This factor is a function of the dielectric constant of the lacquer such that the film weight is computed from the product of the capacitance and this factor K. The value obtained on the gauge is a linear correction of the capacitance measured and allows the measurement of the film weight of this particular lacquer on production sheet metal.
One major problem with this known system is that recalibration is time consuming and needs to be carried out independently for each channel of the gauge, typically 40 or more channels being used. As a panel is required for checking each of the 40+ channels which are programmed, this inevitably leads to infrequent recalibration. These panels are also not produced to a common, traceable standard and are prone to damage and consequent inaccuracy through repeated usage.
A further problem with known gauges is that there is no way of quantifying either the condition of the tip of the probe which is used together with the gauge, or any contamination of the probe tip.